L2-Engineering geophysicssasjsjsjkala.ppt

we

1. Engineering geophysics
 
•Most Geophysicists are trained in exploration
problems which are different from engineering
problem.
•Exploration geophysicists design and conduct
cost effective geophysical surveys to acquire
desired resource in economic quantities.
•The engineering geophysicists are concerned
with resolution, detail and quantification of
specific subsurface condition. It target areas are
usually difficult and success may not be
possible.

2. The major application areas for engineering geophysics :
•Site Assessment - determine bedrock depths
•Correlation - correlation of geological units between
boreholes
•Hazards -location of faults, geological contacts,
shear zones, cavities and man-made objects.
•Engineering - Determination of dynamic elasticity, rippability
Properties
 
•Resources - Location of burrow materials such as sand,
gravel, clay and hard rock
 
•Groundwater - Determination of ground water conditions,
location of leakage zones.
 
•Monitoring - Blasting efectiveness assessment.

3. Role and function of engineering geophysicist.: 
FUNCTION AND OPERATIONS FLOWCHART FOR ENGINEERING GEOPHYSICAL SURVEY
- Budget - interpreted drilling log - Initial physical model - Field
- Problem - Selection of methods - Selection of field procedures operations
- Site information - Initial geological model - geological classifications
- Deadline & timing - Selection of contractor
- Monitoring - Supervision of contractor
 
Civil Engineer Geologist Geophysicist Contractor
 
Completion/evaluation
 
- Critical evaluation - Final geological model - Data assessment - Supply of -
integration report - Physical interpretation field data
- Final Opinion - Correlation - Operation report
- Geolog. & engineering
Interpretation
- Ambiguities and
limitation
- Report
 

4. Common problems in the operation of
engineering geophysical surveys
Civil engineer
•Budget for geophysics inadequate or
inappropriate
•Geophysical survey poorly integrated with
other investigation
•Scale and implication of geophysical
interpretation unclear
•Geophysical interpretation disagree with
other information

5. Engineering geologist
•Poor core recovery in drilled holes
•Geological classification difficult or
inappropriate
•Unresolved ambiguities in geological
model

6. Engineering geophysicist
•Subsurface condition very different from
those expected
•Inadequate and poor quality data
•Interpretative tools inadequate for highly
variable subsurface condition
•Unresolved ambiguity

7. Geophysical contractor

• Site conditions not as expected
• Decreased coverage owing to
equipment failure, weather, working space
etc.
•Other site work interfering with operation

8. Engineering geophysics has an important
role to play in geotechnical investigations
for civil engineering and mine development
projects, however geophysicists need to
realize that greater care is required in
interpretation and reporting geophysical
results.
In particular geophysicists need to develop
an increased sensitivity to alternative
interpretations, consistent with a measured
data set and their engineering implications.

9. Comparison of different interpreted models of seismic refraction interpretation
 
Original Intercept time Intercept time Raytrace Velocity function
Interpretation Re-interpretation re-interpretation interpretation interpretation
With blind zone
340 350
- 0m ======= ------------------ ____350________ __300__________ ___340______
550 800 800 730 800
________
1600 __________ _____________ _____________ ____________
-50 ________ 2200 ___1600______
V(s)= 1650(1+0.01z)
_____________ 2200 2450
­
----------------------
 
-100 3300 3000 3000 _____________

3250
_____________
-150 4800 _______________
4800
 
-200m
 

10. Engineering Geophysics : A Geologist’s view
 
Three remarks given by geologists:
1. Great! Provides quantitative data which our geologists
can’t do. We are saving a lot of drilling
2. Hopeless! Misled us badly on several jobs. We don’t use
it any more.
3. Useful, as an extra tool in site investigation, but requires
very careful interpretation, and calibration.

Case 2 is during time when Seismic refraction survey
was oversold by geophysical companies and engineers
(Late fifties).Today seismic survey mostly yield Case 3

11. Geophysicists usually involved in all the
investigation stages.
Common applications:
• Location of features with anomalous physical
properties
• Delineation of boundaries between features with
contrasting physical properties
• Assessment of physical properties of rock masses
in situ:
o
     
Rippability
o
      
Elastic properties
o
      
Effects of grouting

12. Location of features with anomalous physical
properties:
•Basic dyke/ plugs
•Deeply weathered zone
•Major fault zones
•Buried channels cavities
•Land slipped masses

13. Delineation of boundaries between features with
contrasting physical properties
•Transported overburden
•Residual overburden
•Transported overburden over residual
overburden
Assessment of physical properties of rock masses
in situ
•·
       
Rippability
•·
       
Strength and compressibility
•·
       
Effectiveness of grouting

14. Challenges in future research:
•Location of hazardous features in offshore situations by any
geophysical method, in particular, high reflection seismic
•Location of thin continuous, weak seams in rock masses.
Single seams of this kind have often been the cause of major
rockslides.
•Location and delineation of residual fresh boulders within
masses of extremely weathered rock (residual soil)
•Location and delineation of hidden cavities in rock or soil
masses
•Development of a method for the remote-reading of
instruments (eg. Piezometers, extensometers) installed at great
depth in rock masses. This is required for effective monitoring
of rock mass behaviour during subsidence, during and after
mining activities.
•Assessment of the depth of mechanically loosened rock over
compact rock, both in surface exposures, and around tunnels
and other underground excavations
•Looking of points or areas of leakage from the floor of
reservoirs.